What is the recognized good practice for inerting gaseous hydrogen vent lines?
The recognized and generally accepted good engineering practices (RAGAGEP) for employing a purge into a hydrogen vent stack is that inerting is not generally used. Best practice is that the vent stack should be designed for a fire and the overpressure caused by an internal deflagration. This is typically not an issue for smaller sizes (less than 4”) and when using typical materials for a vent stack (carbon or stainless steel pipe).
CGA G-5.5, Standard for Hydrogen Vent Systems, Section 6.2 states that “Hydrogen vent systems do not require inerting of the vent stack or flare to ignite gases exiting the vent system.” Section 5.5 says a reduced L/D (length over diameter ratio) of the vent line lowers the potential for an explosion in the vent stack. However, CGA G-5.5 also states in Section 6.2.12, “Hydrogen vent systems within the scope of this publication (gaseous and liquid hydrogen at user sites) are unlikely to sustain deflagration or detonations, regardless of the L/D ratios. The relatively simple geometry of the system (few turns, few tie ins) and operating scenarios are not conducive to forming detonable hydrogen-air concentrations within the system and limit potential ignition sources external to the stack discharge. In the unlikely instance that a deflagration or detonation occurs, experience has shown that a system designed for 150 psig (1030 kPa) will sustain the event without bursting.” EIGA Doc 211/17, Hydrogen Vent Systems for Customer Applications, specifically does not cover inerted vent systems in Section 2. It does require the vent stack design to withstand the maximum peak pressure created by detonation unless an inert gas is used for continuous purging. EIGA requires a vent stack design pressure minimum of 40 bar for a maximum detonation pressure of 120 barg (section 5.5). It is important to note that the vent stack should not have an opening in the vent system that can pull air into the vent stack (e.g., an open drain connection), as this substantially increases the risk of a fire, deflagration, or detonation in the vent stack.
Most gases that might be used to purge a vent stack (where it might be applicable) can also potentially freeze and could form a blockage when used on vent systems for LH2 equipment. Care must be taken to prevent this possibility. Purge vent systems can also create back pressure considerations on relief devices and must be adequately designed when used.